A retardation film which is provided with a specific retardation value by stretching and orienting a polymer film is generally used in a liquid crystal display in order to improve its display quality such as color, contrast and view angle. A retardation film made of a polycarbonate-based polymer resin has been generally used for STN liquid crystal applications. The polycarbonate retardation film is also used as a λ/4 film for 3-D polarizing spectacles which are now in growing demand. The film is produced by a casting method. That is, a polycarbonate is dissolved in a solvent such as methylene chloride, and the resulting solution is extruded from a slit die onto a support medium such as a mirrored steel belt or steel drum as a liquid film which is then dried to obtain an unstretched film. Thereafter, the unstretched film is stretched to orient its molecules so as to produce a retardation film.
A film which is uniform in thickness is easily obtained by the casting method as compared with a melt extrusion method. In the melt extrusion method, a resin is thermally deteriorated by heating or shear stress at the time of kneading, whereby foreign matter defects are readily produced in the film, and the film is readily colored. In respect of these points, the casting method is superior to the melt extrusion method and has been preferably used for optical applications including retardation films.
However, the casting method involves many problems to be solved, such as a huge amount of energy required for the drying and collection of a solvent, high equipment cost as the equipment is relatively large in size, high production cost as the production speed is limited by drying time, and concern about environment due to use of methylene chloride which is a halogen-based solvent. Therefore, the production of a retardation film by the melt extrusion method is now under study energetically, and various studies are being made on a thermoplastic norbornene-based resin as in Patent Documents 1 and 2.
Meanwhile, the following proposals have been made on the polycarbonate-based resin. For example, Patent Document 3 discloses a retardation film having a specific retardation value and a specific dispersion width of the average value of retardation, which is obtained by monoaxially stretching a polycarbonate-based film having a specific intrinsic viscosity and a specific dispersion width of intrinsic viscosity.
Patent Document 4 discloses a retardation compensation film which has a specific value or less of high-molecular weight foreign matter content and is made of a polycarbonate having a viscosity average molecular weight of 20,000 to 100,000.
It is proposed in Examples and Comparative Example 1 of Patent Document 5 to obtain a retardation film by melting a polycarbonate having a molecular weight of 15,000, extruding it from a nozzle to obtain a rod and stretching a disk cut out from the rod.
Patent Document 6 proposes a melt extruded film of a polycarbonate which is useful as a film for the light transmitting layer of an optical disk marketed under the trade name of “Blue-Ray disc” and a protective film for the polarizing plate of a liquid crystal display (WO2007/141899). This film is an optically isotropic unstretched film having a small retardation.
The proposal of a stretched retardation film having few defects produced by foreign matter and small nonuniformity in retardation has been unsatisfactory as described above.    (Patent Document 1) JP No. 3273046    (Patent Document 2) JP No. 3407714    (Patent Document 3) JP No. 2841376    (Patent Document 4) JP No. 3203069    (Patent Document 5) JP No. 2612196    (Patent Document 6) WO2007/141899